The Model Driven Architecture (MDA) is an initiative proposed by the Object Management Group (OMG) to model centric software development. It is based on the concepts of models, metamodels and automatic transformations. A crucial part of the evolution from abstract models to executable components or applications is accomplished by means of refactoring. MDA-based refactorings can be specified in the Object Constraint Language (OCL) as contracts between metamodels. We propose an alternative formalization based on the NEREUS language that can be viewed as an intermediate notation open to many formal languages. We show how to transform automatically OCL contracts into NEREUS. We propose a uniform treatment of refactoring at platform independent, platform specific and implementation specific abstraction levels.